Mobile telephony networks (Public Land Mobile Networks, shortly PLMNs) were initially conceived for enabling voice communications, similarly to the wired networks (Public Switched Telephone Networks, PSTNs), but between mobile users. Mobile telephony networks have experienced an enormous spread, especially after the introduction of second-generation mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese corresponding systems).
The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Message Service (SMS) and Multimedia Message Service (MMS) services, and Internet connectivity services have been made available in the last few years.
More recently, 3G mobile communication systems, like those complying to the Universal Mobile Telecommunications System (UMTS), are being deployed, bringing about significantly higher information exchange rates, allowing network operators to offer new services to the mobile users.
However, PLMNs are born as Circuit-Switched (CS) networks and, as such, are more suitable for voice communications than for exchanging relatively large amounts of data. Data communications are better achieved by adopting Packet-Switched (PS) schemes, like in computer networks, particularly the Internet. This remains true also for 3G mobile communications systems, despite their increased communications rate capabilities. The PS domain of the UMTS is constituted by a core network, which is the evolution of the GPRS core network, and a radio access network known as the UTRAN (UMTS Terrestrial Radio Access Network). The UTRAN complying with the 1999 release of the standard (so-called “R99”) is able to support PS transmission up to 384 Kbps for the support of person-to-person or content/network-to-person communications, by means of dedicated connection over radio.
Usually, in PLMNs, even if provided with a UTRAN infrastructure, the information content is transferred in a Point-To-Point (P-T-P) or unicast mode, upon activation of a session between a User Equipment (UE) and a service provider connected to a packet data network, e.g. a server connected to the core network or to the Internet; the activation of such a session involves the setting up of logical and physical connections between the server and the UE. In such a P-T-P communication mode, the radio resources to be allocated for the exchange of data between the network and the UEs depend on the number of different mobile stations simultaneously exploiting the services, even if two or more users take advantage of the same information content at the same time. This limits the possibility of simultaneously accessing available services by several users, unless the radio resources are oversized.
Thus, it is desirable to have the possibility of delivering information contents related to a same service exploitable by two or more users at a time based on a different, Point-To-Multipoint (P-T-M) or multicast/broadcast mode, so as to save the amount of allocated radio resources.
In this respect, the 3GPP (3rd Generation Partnership Project) standardization group is discussing the implementation, both in the GERAN (GSM/EDGE Radio Access Network) and in the UTRAN (UMTS Terrestrial Radio Access Network) frameworks, of a new kind of service architecture, named MBMS (Multimedia Broadcast/Multicast Service). Basically, the MBMS targets the simultaneous distribution of information content (particularly, multimedia content) to more than one mobile user from a single serving base station over a common radio resource; this is for instance the case of short clips of sport matches delivered to UEs of mobile users, or of the television over mobile. In other words, PLMN operators experience the need of proper mechanisms in the network in order to efficiently transport simultaneously the same information content to specified groups of users.
The 3GPP Technical Specification (TS) 25.346 V.6.4.0 (“Introduction of the Multimedia Broadcast Multicast Service (MBMS) in the Radio Access Network (RAN)”, hereinafter also referred to as “the 3GPP standard” or, shortly, “the standard”), relates to the introduction of the MBMS in the Terrestrial Radio Access Network (Terrestrial RAN) of a terrestrial UMTS network (the so-called UTRAN). The document describes techniques for optimized transmission of MBMS bearer service in the UTRAN, such as P-T-M transmission, selective combining and transmission mode selection between P-T-M and P-T-P: an MBMS service may be delivered in either P-T-P or P-T-M mode depending on the number of MBMS users in the generic cell (particularly, if the number of MBMS users in the cell does not exceed a certain operator-defined threshold, e.g. 4 to 6 users, the P-T-P mode is selected). In particular, from the radio point of view, the proposed solution provides for the support of broadcast/multicast services either by means of a plurality of P-T-P channels, each one dedicated to a respective MBMS user, or by means of P-T-M channels, using the downlink SCCPCH (Secondary Common Control Physical CHannel). The SCCPCH is a common physical channel on the downlink path that was already present in the Release 1999 of the standard (3GPP R99), and was intended to be used to carry the Forward Access transport CHannel (FACH), a transport channel that can carry radio signaling (it is mapped to the DCCH—Dedicated Control Channel—and the CCCH—Common Control CHannel) and short, low frequent user data (it is also mapped to the DTCH—Dedicated Traffic CHannel). In order to support P-T-M communications over the UMTS radio interface, in the Release 6 of the standard (3GPP R6) the FACH has been modified so as to allow the transport of three different logical channels, named MCCH (MBMS P-T-M Control Channel), MTCH (MBMS P-T-M Traffic Channel) and MSCH (MBMS P-T-M Scheduling Channel).
A drawback of the solution adopted in the standard for implementing the P-T-M delivery of an MBMS is that the FACH channel is not power controlled, and it is then transmitted at fixed power, or at best its power can be reconfigured in a slow way compared to the closed-loop power control which is instead implemented for normal, P-T-P channels such as the DCHs (Dedicated Channels); for this reason, a certain amount of power should be reserved for the delivery of an MBMS, in principle even when no MBMS users are present in the system; the power required by the FACH allocated for MBMSs may be a significant fraction of the total power. Power is however a scarce and thus precious resource in a system that, like the UMTS, is based on the WCDMA (Wide-band Code Division Multiple Access); it is for this reason that the standard relies on a P-T-P MBMS delivery mode over conventional, power-controlled, P-T-P DCHs when the number of MBMS users in a cell is limited.
In order to improve the downlink power management, the 3GPP standardization forum is introducing a set of combining functions for the SCCPCH channel designed for MBMS in order to further improve the MBMS channel performance. With this feature, when a 64 kbps MBMS channel is provided, the power used by the FACH can go from around 10% up to 20% of the total transmitted power as a function of the Transmission Time Interval (TTI), propagation channel condition, and number of radio links among which the combining is performed (2 or 3 radio links). However, even in this case the delivery of a 64 kbps MBMS requires a dedicated power of about 10-20% of the total transmitted power in a typical indoor environment. As a consequence, the standard solution requires a significant amount of power already needed for MBMS services limited in bit rate (0.1%-1% target BLER—radio BLock Error Ratio).
US 2003/0119452 proposes a method for controlling transmission power of a plurality of UEs by a Node B to perform broadcasting in a mobile communication system. In particular, the three different alternative approaches are described.
According to a first approach, the use of a new common channel is foreseen supporting MBMS and shared by all the MBMS users (named Physical Broadcast Multicast Shared CHannel—PMBSCH). Unlike the other channels, the PBMSCH transmits only pure MBMS data, and not control information. The PMBSCH is used in conjunction with another new channel, named CPCCH (Common Power Control CHannel) used for the power control of the PBMSCH. In case however the number of MBMS users in a certain cell is below a predetermined threshold, a number of DCHs equal to the number of users are allocated to deliver the MBMS contents.
A second approach proposed in the cited document tries to maximizes the efficiency of channelization code resources and transmission power resources by using a single DPDCH (Dedicated Physical Data CHannel) on the downlink for MBMS data transmission, N informal DPCCHs (Dedicated Physical Control CHannels) on the downlink for the transmission of signaling information to the UEs, and N DPCCHs on the uplink. The radio base station uses the N informal DPCCHs on the downlink for transmitting TPC (Transmit Power Control) commands to each single MBMS UE, instructing the UE to increase/decrease the transmission power of the uplink path according to the output of the power control algorithm run in the radio base station.
In a third embodiment proposed in the cited document, a separate power control for the N informal DPCCHs is guaranteed: the transmission power on each single informal DPCCH is dynamically calculated, and, on the basis of this calculation, and particular on the basis of the worst radio channels, the transmission power of the common DPDCH is calculated.